Despite extensive study, fundamental aspects of hepatic fatty acid and organic anion transport remain unexplained. In particular, it is not clear whether the hepatic transport mechanism responds to the unbound or to the much greater protein-bound fraction is plasma. Several competing hypotheses have been proposed to explain existing uptake data, including surface-catalyzed dissociation of the albumin-ligand complex and the presence of an albumin receptor. We have recently proposed that these results may be due to the fact that uptake rates may be determined not only by membrane transport events, but also by the rate of dissociation of these molecules from albumin and the rate of diffusion across unstirred water layers near the cell surface. We have developed the mathematical and computational tools necessary to determine which of these alternative models is correct. The proposed studies will measure dissociation rates of fatty acids, bilirubin and other organic anions from albumin and compare them to the rates of uptake determined in the isolated perfused rat liver system to determine if dissociation is catalyzed by the liver cells. In addition, the effect of varying the albumin concentration on influx and effux rates will be analyzed to determine if albumin interacts with the membrane during uptake and whether diffusional barriers or the rate of dissociation from albumin significantly limit the uptake rate. The effect of varying ion gradients such as Na+ and H+ on the uptake rate will be determined in the intact liver and used to assess the electrochemical forces driving influx and efflux. The latter studies will utilize a new model in which conventional microelectrodes are used to follow the membrane potential difference during ion substitution while influx and efflux rates are monitored simultaneously. The long term objective is to describe in detail the sequence of events that occurs during the hepatic uptake of organic anions. These experiments will help explain the transport defects that occur in cirrhosis and other liver diseases that lead to jaundice and liver failure, and may lead to new therapeutic strategies for treatment of these diseases.